The present invention relates to a switching unit comprising:                a switch core fabric,        an output termination module coupled to said switch core fabric,        an input termination module coupled to said switch core fabric,said output termination module comprising:        an egress queue adapted to backlog data units belonging to a class of service,        an egress scheduler coupled to said egress queue, and adapted to schedule data units from said egress queue towards an output termination port,said input termination module comprising:        an ingress queue adapted to backlog data units belonging to said class of service and bound to said output termination module,said switching unit further comprising, as forming part of said input termination module, of said switch core fabric and of said output termination module, at least point-to-point transmission means adapted to convey data units from at least said ingress queue to at least said egress queue, said at least point-to-point transmission means being considered, with respect to said ingress queue and to said egress queue, as a virtual ingress-to-egress pipe connecting said ingress queue to said egress queue.        
To operate a switching unit efficiently, and to support controlled Quality of Service (QoS) differentiation in a bursty, unpredictable traffic pattern environment, the switching unit has to be complemented with a flow control mechanism. The granularity of the individual amounts of user traffic for which a flow control decision has to be made, the number of destinations and Classes of Service (CoS) to be distinguished, and the state of technology that can be used to implement the flow control, determine the rate and precision of the flow control decisions, and whether this flow control can be realized in a centralized or distributed manner. As the switching unit size and the applied flow granularity grow, either higher processing and communication speed, or more calculation and communication resources are required.
Credit based flow control grants every potential source of traffic in the switching unit a given limited amount of traffic it can allow into the fabric. The source can use the obtained credit at its own discretion. The credit is renewed on demand or on a regular basis. The calculation rule for granting the credit has to be fair over time to avoid giving an unjustified advantage to a particular physical path across the switching unit.
Internal Dynamic Rate-based Flow Control (IDRFC) is a credit based flow control, involving bandwidth negotiation between each and every input and output termination module within a fixed time schedule (IDRFC cycles).
The cited document discloses a switching unit that implements a weighted IDRFC algorithm for sharing an internal excess bandwidth among the input termination modules, according to bandwidth requests from the input termination modules. The bandwidth request and grant related to an input termination module linked by a virtual ingress-to-egress pipe to an output termination module are both calculated for a determined number of relative administrative weights corresponding each to a different CoS, with a different request and a corresponding grant for every weight.
The processing power required for calculating bandwidth requests and grants scales linearly with the number of CoS. The scalability of a switching unit implementing such a flow control is thus limited.